Internet Accounts can be found in System Preferences. Internet Accounts is a System Preference pane, where an account’s details are entered, and settings for existing accounts can be changed.
Once an account’s credentials have been accepted, we’re invited to choose which of the account’s features we’d like to use, and manage though Apple’s suite of internet account applications. Available features will vary from one account provider to another.
The account’s basic features can be accessed through Apple’s Internet Account-related apps:
Note: If you don’t wish to use Apple’s suite of internet account apps, you don’t need to add your account credentials in System Preferences->Internet Accounts!
For example, you may prefer to use Microsoft’s Outlook in manage your email, calendar, contacts… In which case you’d need to add your account credentials in Microsoft Outlook, not in System Preferences->Internet Accounts.